Combined multiplexer and switched gain circuit

ABSTRACT

A combined multiplexer and switched gain circuit ( 250 ) that selectively multiplexes differential analog signals from a primary channel ( 20 ) and a diversity channel ( 22 ) in a diversity receiver system ( 10 ) to a single output. The circuit ( 250 ) is based on a current mode logic design where a plurality of separate conduction paths ( 278 - 284 ) are provided between a voltage line ( 266 ) and a current source ( 268 ). An output line ( 264 ) of the circuit ( 250 ) is coupled to each conduction path ( 278 - 284 ) so that the differential analog signals from the primary channel ( 20 ) and the diversity channel ( 22 ) can be selectively outputted to the circuit ( 250 ). Each conduction path ( 278 - 284 ) includes a gain device, such as degenerative resistor, that provides signal gain or no signal gain for that conduction path ( 278 - 284 ). Control signals are selectively applied to switching devices ( 310 - 324 ) and each conduction path ( 278 - 284 ) so that the conduction path ( 278 - 284 ) can be independently selected to provide the multiplexing.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to a multiplexer formultiplexing analog signals from a primary channel and a diversitychannel in a diversity receiver and, more particularly, to a combinedmultiplexer and switched gain circuit for multiplexing differentialanalog signals from a primary channel and a diversity channel in adiversity receiver, where the multiplexer provides a gain path and ano-gain path for both the primary channel signals and the diversitychannel signals.

[0003] 2. Discussion of the Related Art

[0004] Historically, communications systems sampled analog signals toprovide signal processing in the system. Modern trends generallyrepresent signals in communications systems as time sampled digital datasignals. Because of the availability of very high frequency circuits, ithas become possible to process digital signals at higher and higherfrequencies into the 100's of MHz. Digital signal processing offers theadvantages of flexibility for change, absolute accuracy without the needfor calibration of analog components that are subjected to temperatureand aging effects, and the ability to perform very complex signalprocessing at modest costs. Further, digital signal processing offersthe possibility of sharing circuit components for multiple tasks,further reducing system hardware and related costs. However, digitalcircuit components become very expensive at high frequencies. Further,for those digital systems that process both radio frequency (RF) andintermediate frequency (IF) signals, extreme care must be taken tomaintain the accuracy of the signal, especially for waveforms employingcomplex modulation.

[0005] Cellular telephone base stations employ many receiver circuitsfor receiving and processing cellular telephone signals. Each receivercircuit typically employs two channels, a primary channel and adiversity channel, each having a separate antenna, so that the receivercircuit can select which of the two received signals is the strongestfor subsequent processing. Some receiver circuits combine the primarychannel and diversity channel signals for increased performance. Thisallows the receiver to be more reliable by lessening the chance thatcellular calls are dropped. However, receivers of this type have beenlimited in their effectiveness for reducing circuit components, reducingthe size and cost of IF sampling circuits, and maintaining signalfidelity at high frequencies.

[0006] Receiver circuits for cellular base stations employ automaticgain control (AGC) using variable gain amplifiers (VGAs) andmultiplexers for amplifying and selecting analog signals propagatingthrough the primary and diversity channels. The analog signals are alsoapplied to an analog-to-digital converter (ADC) to be converted todigital signals for digital processing. One or more ADCs are employed atvarious locations in the circuit relative to the VGA and multiplexer.

[0007] The known Analog Devices AD6600 Diversity Receiver chipsetprovides independent channel attenuation, multiplexing, signal gain andanalog-to-digital conversion on a single chip. In this design, an analogmultiplexer is preceded by individual variable gain attenuator stagesfor each channel and a peak detecting gain control circuit.

[0008] U.S. Pat. No. 5,861,831 discloses a clock-to-clock auto-rangingADC that operates directly on an analog signal in the IF band or higherto track it's gain range on a clock-to-clock basis to produce a digitalsignal that maintains high resolution of the analog signal withoutclipping or loss of signal sensitivity. The ADC samples the analogsignal at sufficiently high frequency so that a peak detector canaccurately determine the maximum signal level over at least one-half ofa signal period, and then reset the signal gain going into the ADC priorto the beginning of the next sample period. The '831 patent offersimprovements for analog-to-digital conversion, but does not address themultiplexing architecture for high frequency. Further, combining all ofthe functions as is done in the '831 patent onto a single chip slows thespeed of the signal throughput and compromises the isolation between theprimary and diversity channels.

[0009] National Semiconductor has a diversity receiver chipset of thetype being discussed herein that operates at high frequency, butrequires several separate chips in various architectures. Further, thischipset does not multiplex the analog signals, but has a separateparallel VGA and ADC path for each primary and diversity channel. Thisdesign provides good isolation, but drives up the cost ofimplementation.

SUMMARY OF THE INVENTION

[0010] In accordance with the teachings of the present invention, acombined multiplexer and switched gain circuit is disclosed thatselectively multiplexes differential analog signals from a primarychannel and a diversity channel in a diversity receiver to a singleoutput. The circuit is based on a current mode logic design where aplurality of separate conduction paths are provided between a voltageline and a current source. An output of the circuit is coupled to eachconduction path so that the differential analog signals from the primarychannel and the diversity channel can be selectively outputted from thecircuit. Each conduction path includes a gain device, such as adegenerative resistor, that provides signal gain or no signal gain forthat conduction path. Control signals are selectively applied toswitching devices in each conduction path so that a particularconduction path can be independently selected to provide themultiplexing.

[0011] In one embodiment, the switching devices are heterojunctionbipolar transistors. The differential analog signals in the primarychannel are applied to the base terminal of bipolar transistors in twoof the conduction paths, and the differential analog signals in thediversity channel are applied to the base terminal of bipolartransistors in two of the other conduction paths. One of the primarychannel conduction paths and one of the diversity channel conductionpaths provide signal gain, and the other primary channel conduction pathand the other diversity channel conduction path provide no signal gain.A separate control signal is applied to the base terminal of otherbipolar transistors in each conduction path so that a particularconduction path can be selected to provide current flow and cause theanalog signal applied to that conduction path to be provided on theoutput.

[0012] Additional objects, advantages and features of the presentinvention will become apparent from the following description andappended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic diagram of a diversity receiver for acellular telephone base station, where the receiver employs a variablegain amplifier/multiplexer RF integrated circuit, according to anembodiment of the present invention;

[0014]FIG. 2 is a detailed schematic diagram of the variable gainamplifier/multiplexer integrated circuit of FIG. 1 separated from thereceiver;

[0015]FIG. 3 is a schematic diagram of a known analog multiplexercircuit employing bipolar transistors;

[0016]FIG. 4 is a schematic diagram of a known dual switched gaincircuit employing bipolar transistors;

[0017]FIG. 5 is a schematic diagram of an analog multiplexer/switchedgain circuit that provides signal gain and is applicable for themultiplexer shown in FIG. 2, according to an embodiment of the presentinvention;

[0018]FIG. 6 is a schematic diagram of a quad switched gain circuitapplicable to be used in each channel of the receiver shown in FIG. 1,according to an embodiment of the present invention; and

[0019]FIG. 7 is a schematic diagram of a differential PIN diodeattenuator applicable to be used in each channel of the receiver shownin FIG. 1, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The following discussion of the invention directed to a variablegain amplifier and multiplexer provided on a common integrated circuitchip for a dual diversity receiver system is merely exemplary in nature,and is in no way intended to limit the invention or its applications oruses. For example, the multiplexer and amplifier are employed inconjunction with a diversity receiver system in a cellular base station.However, as will appreciated by those skilled in the art, the amplifierand multiplexer of the present invention can be employed in other analogor digital processing systems.

[0021]FIG. 1 is a schematic block diagram of a diversity receiver system10 for a cellular telephone base station, according to an embodiment ofthe present invention. As will be discussed in detail below, the system10 employs a VGA 12 and an analog multiplexer 14 formed on a common RFintegrated circuit (IC) chip 16, according to an embodiment of thepresent invention. The system 10 includes a primary channel 20 and adiversity channel 22 both receiving the same analog cellular signal froman antenna 18 and 24, respectively, to provide system redundancy forreliability purposes. The received high frequency analog signal isfiltered by an image filter 26 and is amplified by a low noise amplifier(LNA) 28 in the primary channel 20, and is filtered by an image filter30 and is amplified by an LNA 32 in the diversity channel 22.

[0022] The primary channel 20 includes a mixer 34 that receives thesignal from the antenna 18 and a lower frequency signal from a localoscillator (LO) 36 to down-convert the high frequency analog signal toan IF signal suitable for subsequent processing in a manner that is wellunderstood to those skilled in the art. Likewise, the diversity channel22 includes a mixer 30 that receives the signal from the antenna 24 andthe signal from the LO 36 to down-convert the high frequency signal toan IF signal for the same purpose.

[0023] The IF signal in the primary channel 20 is amplified by anamplifier 40 and is filtered by a bandpass filter 42. The IF signal inthe diversity channel 22 is amplified by an amplifier 44 and is filteredby a bandpass filter 48. The amplifiers 40 and 44 amplify the IF signalsto suitable levels for subsequent processing. In this embodiment, thefilters 42 and 48 convert the single input signals to differentialoutput signals. In an alternate embodiment, the signals could bedirected off-chip to be converted to differential signals by suitablecircuitry (not shown), such as a transformer. A differential signal is asignal that is split into two parts that are 180° out of phase with eachother and combine to form the complete signal. As is known in the art,differential signals are sometimes generated in communications systemsof this type to provide greater noise immunity.

[0024] The differential IF signal in the primary channel 20 is appliedto a step attenuator 50 and the differential IF signal in the diversitychannel 22 is applied to a step attenuator 52. The attenuators 50 and 52provide a reduction in signal gain if the magnitude of the receivedsignal is too high for the system components. The attenuators 50 and 52can provide any suitable level of attenuation for a particular system.In this embodiment, the attenuators 50 and 52 receive a control signalfrom a digital signal processor 46, discussed in more detail below, thatattenuates the signals to a lower power level, if necessary, to preventsystem saturation and part damage in the event that the received analogsignal is too strong. The attenuators 50 and 52 can be any attenuatorsuitable for the purposes described herein, such as a PIN diodeattenuator. One suitable PIN diode attenuator, according to theinvention, will be discussed below with reference to FIG. 7.

[0025] The differential signal from the attenuator 50 in the primarychannel 20 is applied to a gain trim device 54, and the differentialsignal from the attenuator 52 in the diversity channel 22 is applied toa gain trim device 56. The devices 54 and 56 provide an attenuation or again to the signal so that insertion losses and gain variations in thesystem components caused by variations in manufacturing can becalibrated. In this embodiment, the devices 54 and 56 provideattenuation and gain in 1 dB steps from −7.5 dB to +7.5 dB. However, aswill be appreciated by those skilled in the art, these values areapplication specific. The devices 54 and 56 can be any device suitablefor the purposes described herein, such as a switched gain device. Onesuitable device will be discussed in detail below with reference to FIG.6. The differential signals are then applied to a Nyquist filter 58 inthe primary channel 20 and a Nyquist filter 60 in the diversity channel22.

[0026] As shown, the differential signals in both channels 20 and 22 arethen applied to the multiplexer 14. The multiplexer 14 sequentiallyselects the differential signals in the primary channel 20 and thediversity channel 22 to be output therefrom. Particularly, a clocksignal, 52 MHz in one embodiment, causes the multiplexer 14 toconsecutively select one of the inputs to be the output of themultiplexer 14. Thus, the output of the multiplexer 14 is an analogsignal that includes data from one of the channels 20 or 22 at a giveninstant in time. According to the invention, the multiplexer 14 alsoselectively provides signal amplification. The analog signal is thenamplified by the VGA 12 to provide automatic gain control (AGC).According to the invention, the multiplexer 14 is positioned before theVGA 12 to reduce part count. Particularly, only one VGA is requiredbecause the multiplexer 14 outputs a single analog signal. Themultiplexer 14 and the VGA 12 will be discussed in more detail below.

[0027] The selected and amplified signal is then sent to an ADC 62 to beconverted to a digital signal as is required by the processor 46. Thedigital signal is then sent to the digital processor 46 for processingconsistent with the discussion herein. The processor 46 processes thedigital data stream from the ADC 62, and from this information providesa variety of control signals to the system 10. Particularly, as will bediscussed in detail below, the processor 46 provides a control signal tothe step attenuators 50 and 52 to determine whether to provide theattenuation or not based on signal strength. Further, the processor 46provides a control signal to the gain trim devices 54 and 56 todetermine how much gain or attenuation to apply to the differentialanalog signals for proper calibration. Also, the processor 46 providescontrol signals to the multiplexer 14 to provide a selection signal andgain signal for the output therefrom. Additionally, the processor 46provides a selective gain signal to the VGA 12 to determine the amountof gain provided therefrom.

[0028]FIG. 2 is a schematic diagram of the RFIC chip 16 separated fromthe system 10. The multiplexer 14 includes an amplified path 72,including a differential amplifier 74, and a non-amplified path 76 thatreceive the differential signals from the primary channel 20. Further,the multiplexer 14 includes an amplified path 78, including an amplifier80, and a non-amplified path 82 that receive the differential signalsfrom the diversity channel 22. In this embodiment, the amplified paths72 and 78 amplify the differential signals by +12 dB. However, this isby way of a non-limiting example, in that other designs may employdifferent levels of gain. As shown, the multiplexer 14 is selecting theamplified path 72.

[0029] The VGA 12 includes a first amplifier stage 88 having anamplified path 90 and a non-amplified path 92, where an amplifier 94 isprovided in the amplified path 90. The VGA 12 also includes a secondamplifier stage 96 including an amplified path 98 and a non-amplifiedpath 100, where the amplified path 98 includes a differential amplifier102. As will be discussed in detail below, the first amplifier stage 88applies either a +12 dB gain or no gain to the signal selected by themultiplexer 14, and the second amplifier stage 96 provides a +6 dB gainor no gain to the signal selected by the multiplexer 14. Thus, bycombining the gained choices of the various paths of the combination ofthe multiplexer 14 and the VGA 12 as discussed herein, the differentialsignal on either the primary channel 20 or the diversity channel 22 canadd either 0, +6, +12, +18, +24, +30 dB gain at the output of the RFICchip 16. These gains are application specific and can be other gainchoices in other embodiments within the scope of the present invention.

[0030] Gain selection control signals are applied to a low volttransistor-transistor logic (LVTTL) circuit 104. Particularly, an AGCSEL digital control line from the processor 46 selects the primarychannel 20 or the secondary channel 22, and AGC0, AGC1 and AGC2 digitalcontrol lines from the processor 46 determine which gain the RFIC chip16 will provide to the differential signal selected. The circuit 104includes any suitable digital logic components that decode the digitalcontrol signals consistent with the discussion herein. The decodedcontrol signal from the circuit 104 is sent to a latch control circuit106 including a series of flip-flops (not shown) that hold the selectcontrol signal for each clock cycle. An output of the latch controlcircuit 106 is applied to the multiplexer 14, the first amplifier stage88 and the second amplifier stage 96 to select one of the amplified pathor non-amplified path in each component or stage. The proper load, forexample 600 ohms, must be applied to the output of the RFIC chip 16 sothat the desired gain of the analog differential signal is achieved.

[0031]FIG. 3 is a schematic diagram of a known multiplexer 110. Thedesign of the multiplexer 110 employs current mode logic (CML) in a celldesign. The multiplexer 110 further employs heterojunction bipolartransistors (HBTs) that are optimized for wide band signals (DC to 2.5GHZ), switching speed (100 MHz) and linearity (typically third orderintercept of 30 dB). Other designs consistent with the invention can beimplemented in various transistor technologies, including Si bipolar,SiGe HBT, GaAs HBT, InP HBT, and CMOS FET or other FET technologies.

[0032] The multiplexer 110 receives four differential analog inputsignals on differential input lines 112, 114, 116 and 118, and selectsone of the input signals to be provided on differential output lines120. Also, four digital control lines 122, 124, 126 and 128 are coupledto the multiplexer 110 that provide the select function. The multiplexer110 receives a voltage potential on line 130 that is applied toresistors R₁ and R₂. Further, a current source 132 is coupled to outputreference ports 134 and 136, and includes resistors R₃ and R₄ andbipolar transistors 138 and 140. In order to operate properly, thevalues of the resistors R₁ and R₂ and the values of the resistors R₃ andR₄ must be the same or nearly the same value.

[0033] As will be discussed below, the multiplexer 110 selects whichdifferential input to transfer to the output lines 120 by providingcurrent conduction from the voltage line 130 to the current source 132through one of four conduction paths 142, 144, 146 and 148. Eachconduction path 142-148 includes two lines that are coupled to theoutput lines 120. Four sets of two bipolar transistors must conduct toprovide current flow through the lines of a particular conduction paths142-148. A bipolar transistor is caused to turn on or conduct byapplying a suitable DC bias to it's base terminal. The lines of theconduction path 142-148 are interconnected as shown to provide currentconduction from the line 130 to the current source 132.

[0034] The input lines 112 are coupled to the base terminal of bipolartransistors 150 and 152 in the conduction path 142. The input lines 114are coupled to the base terminal of bipolar transistors 154 and 156 inthe conduction path 144. The input lines 116 are coupled to the baseterminal of bipolar transistors 158 and 160 in the conduction path 146.The input lines 118 are coupled to the base terminal of bipolartransistors 162 and 164 in the conduction path 148. The control line 122is coupled to the base terminal of bipolar transistors 166 and 168 inthe conduction path 144 and the base terminal of bipolar transistors 170and 172 in the conduction path 148. The control line 124 is coupled tothe base terminal of bipolar transistors 174 and 176 in the conductionpath 142 and the base terminal of bipolar transistors 178 and 180 in theconduction path 146. The control line 126 is coupled to the baseterminal of bipolar transistors 182 and 184 in the conduction path 142.The control line 128 is coupled to the base terminal of bipolartransistors 186 and 188 in the conduction path 146.

[0035] There will always be a DC bias signal on the input lines 112-118whether or not the RF input signal is applied to the lines 112-118.Therefore, any of the transistors 150-164 will be able to conduct theirassociated conduction path given the control lines in that path areproperly configured. Thus, the control lines 122-128 determine whichconduction path 142-148 will conduct to provide the signal selectionprocess. To select the input lines 112, the conduction path 142 iscaused to conduct by providing a logical high signal on the controllines 124 and 126 to turn on the bipolar transistors 174, 176, 182 and184. To select the input lines 114, the conduction path 144 is caused toconduct by providing a logical high signal on the control lines 122 and126 to turn on the bipolar transistors 166, 168, 182 and 184. To selectthe input lines 116, the conduction path 146 is caused to conduct byproviding a logical high signal on the control lines 124 and 128 to turnon the bipolar transistors 178, 180, 186 and 188. To select the inputlines 118, the conduction path 148 is caused to conduct by providing alogical high signal on the control lines 122 and 128 to turn on thebipolar transistors 170, 172, 186 and 188. It will be apparent to thoseskilled in the art how this cell architecture can be expanded to selectsignals on more than four differential input lines.

[0036]FIG. 4 is a schematic diagram of a known dual switched gaincircuit 200 that is applicable to selectively amplify a differentialsignal applied thereto for two different gains. In one embodiment, thecircuit 200 can be used for the amplifier stages 88 and 96. The CMLarchitecture of the gain circuit 200 is based on the same principles asthe multiplexer 110 above. The gain circuit 200 includes a pair ofdifferential signal input lines 202, a pair of differential output lines204, a first control line 206, a second control line 208 and a voltageline 210 coupled to resistors R₁ and R₂. The gain circuit 200 alsoincludes a current source 212 having bipolar transistors 214 and 216 andresistors R₅ and R₆ coupled to voltage reference ports 218 and 240. Thegain circuit 200 further includes a first gain conduction path 220 and asecond gain conduction path 222 between the voltage line 210 and thecurrent source 212. Both conduction paths 220 and 222 are coupled to theoutput lines 204.

[0037] The gain paths 220 and 222 provide a different gain as set bydegenerative resistors R₃ and R₄ respectively. The resistors R₃ and R₄are referred to as degenerative because they are tied to the emitterterminal of a bipolar transistor set (e.g., R₃ to transistors 224 and226, R₄ to transistors 228 and 230) of a Gilbert Mixer typearchitecture. A transfer function determined by the ratio of the valueof the resistor R₃ to the values of the resistors R₁ and R₂ determinesthe gain of the conduction path 220. Likewise, the ratio of the value ofthe resistor R₄ to the values of the resistors R₁ and R₂ determines thegain of the conduction path 222. The transfer function is defined by2R_(L)/(R_(G)+2r_(e)), where R_(L) is R₁ or R₂ (since they areidentical), R_(G) is the path gain degenerative resistor, and r_(e) isthe emitter resistance of the respective bipolar transistor. If R_(G) isproportionately less than R_(L) then the conduction path provides gain,and if R_(G) is proportionately greater than R_(L) then the conductionpath provides attenuation. If the gain circuit 200 is used for theamplifier stage 88 or 96, then the value of one of the resistors R₃ orR₄ will be selected such that the associated conduction path provides again of one, and passes the input signal through unchanged. Alternately,the other resistor will be sized to provide the desired gain. Thenon-amplified path 92 or 100 would be the conduction path that does notprovide gain, and the amplified path 90 or 98 would be the conductionpath that provided gain.

[0038] The input lines 202 are coupled to the base terminal of bipolartransistors 224 and 226 in the conduction path 220 and the base terminalof bipolar transistors 228 and 230 in the conduction path 222. Thedifferential analog input signal is continually applied to the inputlines 202 and the bipolar transistors 224, 226, 228 and 230 are allproperly DC biased since the stages are DC coupled. Therefore, thecontrol lines 206 and 208 determine which path 220 or 222 will conduct.Particularly, a logical high signal on the control line 206 is appliedto the base terminal of bipolar transistors 232 and 234 to turn thetransistors 232 and 234 on and cause the conduction path 220 to conduct.Likewise, a logical high signal on the control line 208 is applied tothe base terminal of bipolar transistors 236 and 238 to turn thetransistors 236 and 238 on and cause the conduction path 222 to conduct.The amplified input signal from the conducting path is provided on theoutput lines 204.

[0039]FIG. 5 is a schematic diagram of a multiplexer/switched gaincircuit 250, according to an embodiment of the present invention, thatcan be used for the multiplexer 14 discussed above. The design of thecircuit 250 combines features of the multiplexer 110 and the gaincircuit 200. The circuit 250 includes first differential input lines252, second differential input lines 254, a first digital control line256, a second digital control line 258, a third digital control line260, a fourth digital control line 262, differential output lines 264, avoltage line 266 and a current source 268. The voltage line 266 iscoupled to resistors R₁ and R₂ in the same manner as discussed above.Likewise, the current source 268 includes resistors R₇ and R₈ andbipolar transistors 272 and 274 coupled to output reference ports 270and 276. The circuit 250 defines four gain conduction paths between thevoltage line 266 and the current source 268, including a first gainconduction path 278, a second gain conduction path 280, a third gainconduction path 282 and a fourth gain conduction path 284. Degenerativeresistors R₃, R₄, R₅ and R₆ in the respective conduction path 278-284determine the gain (or no gain) for that path. Each conduction path278-284 is coupled to the output lines 264.

[0040] The first differential input lines 252 are coupled to the baseterminal of bipolar transistor 288 and 290 in the first conduction path278 and to the base terminal of bipolar transistors 292 and 294 in theconduction path 280. The second differential input lines 254 are coupledto the base terminal of bipolar transistors 296 and 298 in theconduction path 282 and to the base terminal of bipolar transistors 300and 302 in the conduction path 284. The control line 256 is coupled tothe base terminal of bipolar transistors 310 and 312 in the conductionpath 278. The control line 258 is coupled to the base terminal ofbipolar transistors 314 and 316 in the conduction path 280. The controlline 260 is coupled to the base terminal of bipolar transistors 318 and320 in the conduction path 282. The control line 262 is coupled to thebase terminal of bipolar transistors 322 and 324 in the conduction path284.

[0041] A DC bias is applied to the base terminal of the transistors288-302 so that they can be turned on. The control lines 256-262determine which conduction path 278-284 is selected to select the inputsignal with the desired gain. As discussed herein, a logical high on onecontrol line 256-262 means that a higher DC voltage is applied to thatcontrol line relative to the other control lines. A logical high signalon the control line 256 causes the conduction path 278 to conduct toprovide the input signal on the lines 252 to be output on the outputlines 264 with the gain determined by the resistor R₃. A logical highsignal on the control line 258 causes the conduction path 280 to conductto provide the input signal on the lines 252 to be output on the outputlines 264 with the gain determined by the resistor R₄. A logical highsignal on the control line 260 causes the conduction path 282 to conductto provide the input signal on the lines 254 to be output on the outputlines 264 with the gain determined by the resistor R₅. A logical highsignal on the control line 262 causes the conduction path 284 to conductto provide the input signal on the lines 254 to be output on the outputlines 264 with the gain determined by the resistor R₆.

[0042] If the circuit 250 is used for the multiplexer 14, then one ofthe conduction paths for both input signals would have no gain. Morespecifically, in order to provide the non-amplified path 76, one of theresistors R₃ or R₄ would provide a no-gain conduction path for the inputsignal on lines 252. Likewise, in order to provide the non-amplifiedpath 82, one of the resistors R₅ or R₆ would provide a no-gainconduction path for the input signal on lines 254.

[0043] The gain circuit 250 provides certain advantages includinglimited power consumption because a single current source is employed.Further, other gain conduction paths can be added that only requireadding more control lines and transistors without the need foradditional power. Although each input has two gain paths in this design,other embodiments can employ more gain paths for each input. Further,different numbers of gain paths can be provided for each input.

[0044]FIG. 6 is a schematic diagram of a quad switched gain circuit 350,according to an embodiment of the present invention, based on the CMLcell architecture discussed herein. The gain circuit 350 receives adifferential analog input signal on input lines 352 and applies a gainor attenuation thereto based on a selected gain configuration. In oneembodiment, the gain circuit 350 has particular application for the gaintrim devices 54 and 56 to provide 1 dB changes in attenuation or gainbetween −7.5 dB and +7.5 dB. However, as will be appreciated by thoseskilled in the art, the gain circuit 350 has application for othercircuits, such as a transmitter circuit.

[0045] The circuit 350 includes a first control line 356, a secondcontrol line 358, a third control line 360, a fourth control line 362,differential output lines 364, a voltage line 366 and a current source368. The voltage line 366 is coupled to resistors R₁ and R₂ in the samemanner as discussed above. Likewise, the current source 368 includesresistors R₇ and R₈ and bipolar transistors 372 and 374 that are coupledto reference ports 354 and 370. The circuit 350 defines four conductionpaths between the voltage line 366 and the current source 368, includinga first conduction path 378, a second conduction path 380, a thirdconduction path 382 and a fourth conduction path 384. Degenerativeresistors R₃, R₄, R₅ and R₆ in the respective conduction path 378-384determine the gain for that path. Each conduction path 378-384 iscoupled to the output lines 364.

[0046] The differential input lines 352 are coupled to the base terminalof bipolar transistors 388 and 390 in the conduction path 378, to thebase terminal of bipolar transistors 392 and 394 in the conduction path380, to the base terminal of bipolar transistors 396 and 398 in theconduction path 382 and to the base terminal of bipolar transistors 400and 402 in the conduction path 384. The control line 356 is coupled tothe base terminal of bipolar transistors 410 and 412 in the conductionpath 378. The control line 358 is coupled to the base terminal ofbipolar transistors 414 and 416 in the conduction path 380. The controlline 260 is coupled to the base terminal of bipolar transistors 418 and420 in the conduction path 382. The control line 362 is coupled to thebase terminal of bipolar transistors 422 and 424 in the conduction path384.

[0047] A suitable DC bias is applied to the base terminals of thetransistors 388-402 so that the transistors 388-402 can be turned on.The control lines 356362 determine which conduction path 378-384 isselected to set the desired gain or attenuation for the input signal. Alogical high signal on the control line 356 causes the conduction path378 to conduct to provide the gain or attenuation determined by theresistor R₃ to be provided on the output lines 364. A logical highsignal on the control line 358 causes the conduction path 380 to conductto provide the gain or attenuation determined by the resistor R₄ to beprovided on the output lines 364. A logical high signal on the controlline 360 causes the conduction path 382 to conduct to provide the gainor attenuation determined by the resistor R₅ to be provided on theoutput lines 364. A logical high signal on the control line 362 causesthe conduction path 384 to conduct to provide the gain or attenuationdetermined by the resistor R₆ to be provided on the output lines 364.

[0048] As discussed above, the ratio of the values of the resistors R₁and R₂ to the value of the degenerative resistor in the particularconduction path 378-384 determines whether the conduction path 378-384will provide gain or attenuation. For the embodiment discussed herein,gain or attenuation is provided in 1 dB steps between −7.5 dB and +7.5dB (16 steps). As is apparent, the circuit 350 only provides fourvariations in gain or attenuation. In order to get the sixteen steps ofgain, it is necessary to provide another quad switched gain circuitcascaded with the circuit 350. Particularly, a second switched gaincircuit would be coupled to the output lines 364 so that the analoginput signal applied to the input lines 352 goes through two conductionpaths to provide the desired gain or attenuation. Each conduction pathin both switched circuits would have a different resistive value. One ofordinary skill would readily understand how to determine those resistivevalues to provide the sixteen 1 dB steps.

[0049] In an alternate embodiment, the circuit 350 can have sixteensections or conduction paths to provide the desired sixteen 1 dB stepsof gain. However, such a design would require sixteen control lines,thus increasing the control circuitry. Other designs within the scope ofthe present invention can employ switched circuits having more or lesssections and more or less coupled switched circuits to provide differentvariations in dB steps as well as a wider or lesser gain and attenuationrange.

[0050]FIG. 7 is a schematic diagram of a differential PIN diodeattenuator 450 that can be used for either of the step attenuators 50 or52 discussed above, according to an embodiment of the present invention.As will become apparent from the discussion below, the attenuator 450includes all of it's components on a single integrated circuit chipbecause it eliminates the inductors employed in known differential PINdiode attenuator designs. The inductors were necessary in the knowndesigns to prevent the RF input signals from entering the DC controlbias line. Particularly, the conventional approach employed a stacked PIpad configuration using four inductors to block RF from the externalbias/control circuitry. At lower RF frequencies, such as 100-500 MHz,these inductors are too large to be manufactured on an RF LSI chip.Thus, eight input/output pads would be needed to go off-chip and backon-chip for the inductors.

[0051] The differential signal on the input line 452 is directed to anattenuation line 456 or a non-attenuation line 458, and then to anoutput line 460. The attenuation line 456 includes a resistor R₁ thatcombines with resistors R₅ and R₇ in a voltage divider network toprovide the attenuation. The non-attenuation line 458 includes a PINdiode 462 that passes the signal unattenuated when it is biased.Likewise, the differential signal on the input line 454 is directed toan attenuation line 466 or a non-attenuation line 468, and then to anoutput line 470. The attenuation line 466 includes a resistor R₂ thatcombines with resistors R₆ and R₈ in a voltage divider network toprovide the attenuation. The non-attenuation line 468 includes a PINdiode 472 that passes the signal unattenuated when it is biased. Alogical high signal on a control line 474 and a logical low signal on acontrol line 478 biases the diodes 462 and 472 to select thenon-attenuation lines 458 and 468. A logical low signal on the controlline 474 and a logical high signal on the control line 478 removes thebias on the diodes 462 and 472 so that the non-attenuation lines 458 and468 are open circuits to the RF signal, and thus the signals go throughthe attenuation lines 456 and 466.

[0052] Because the input signal is a differential signal, the signals onthe input lines 452 and 454 are 180° out of phase with each other. Whenthe signals propagate down the lines 452 and 454, they enter line 482.When the signals reach the halfway point between the lines 452 and 454at node 484 they cancel each other out. Therefore, by coupling thecontrol line 474 to the node 484, no RF signal enters the control line474. Likewise, the control line 478 is coupled to node 486 halfwaybetween the output lines 460 and 470 so that no RF signal enters thecontrol line 478. Thus, no RF blocking components, such as inductors,are required to protect the integrity of the DC bias control signal.

[0053] Because the attenuator 450 could provide a relatively largeamount of attenuation, R₁ and R₂ would typically need to be large toprovide the desired attenuation. However, as R₁ and R₂ get bigger theybegin to compete with the open circuit provided by the diodes 462 and472 in the attenuation mode, reducing the effectiveness of theattenuation and decreasing the effective bandwidth of the signal. Toovercome this problem, the present invention proposes employing a shuntdiode 490 in the line 456 and a shunt diode 492 in the line 466. Whenthe control line 478 is high to select the attenuation lines 456 and466, the diode 490 is biased, and the ratio of the shunt resistancecaused by the parallel resistors R₅ and R₇ to the series resistor R₁provides the attenuation. Likewise, when the control line 478 is high toselect the attenuation lines 456 and 466, the diode 492 is biased, andthe ratio of the shunt resistance provided by the parallel resistors R₆and R₈ to the series resistor R₂ provides the attenuation. Thisconfiguration allows the resistors R₁ and R₂ to be relatively small andstill provide a large attenuation. Resistor R₁₁ provides more currentlimiting capability when the diodes 490 and 492 are biased.

[0054] When the attenuator 450 is in the non-attenuation mode, the inputimpedance is equal to the parallel combination of resistors R₃ and R₄and resistors R₅ and R₆. However, when the attenuator 450 is in theattenuation mode, the resistors R₁ and R₂ in the attenuation lines 456and 466 drive up the input impedance. In one embodiment, the inputimpedance in the non-attenuation mode is about 200 ohms and the inputimpedance in the attenuation mode is about 400 ohms.

[0055] Circuitry is provided to match the impedance of the attenuator450 to the impedance of the rest of the system for both the attenuationmodes, according to the invention. Particularly, the attenuator 450includes an impedance matching network 496 including resistors R₉ andR₁₀ and diodes 498 and 500. When a high signal is provided on thecontrol line 478 for the attenuation condition, a high signal is alsoprovided on control line 502 to bias the diodes 498 and 500 and couplethe resistors R₉ and R₁₀ in the circuit to change the input impedance.In this embodiment, the control lines 478 and 502 are separate inputs toprovide better power control. However, in an alternate design, thecontrol lines 478 and 502 can be tied to the same line because they bothgo high at the same time. Capacitors C₁-C₄ are DC blocking capacitorsthat prevent DC signals from disturbing the RF input and output signalsof the attenuator 450.

[0056] The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A multiplexer and switched gain circuit formultiplexing differential analog signals from a first channel and asecond channel, both of the first channel and the second channeldifferential analog signals include a first part and a second part, saidcircuit comprising: an output line; a current source for providing asource of current, a first conduction path coupling the current sourceand the output line, said first conduction path including a plurality ofswitching devices and at least one gain device, wherein the first partof the first channel differential signal is coupled to a first switchingdevice in the first conduction path and the second part of the firstchannel differential signal is coupled to a second switching device inthe first conduction path, and wherein a first control line is coupledto at least one other switching device in the first conduction path,said first control line providing a control signal to turn on the firstconduction path to transfer the first channel differential signal to theoutput line with a gain provided by the at least one gain device in thefirst conduction path; and a second conduction path coupling the currentsource and the output line, said second conduction path including aplurality of switching devices and at least one gain device, wherein thefirst part of the second channel differential signal is coupled to athird switching device in the second conduction path and the second partof the second channel differential signal is coupled to a fourthswitching device in the second conduction path, and wherein a secondcontrol line is coupled to at least one other switching device in thesecond conduction path, said second control line providing a controlsignal to turn on the second conduction path to transfer the secondchannel differential signal to the output line with a gain provided bythe at least one gain device in the second conduction path.
 2. Thecircuit according to claim 1 further comprising a third conduction pathcoupling the current source and the output line, said third conductionpath including a plurality of switching devices and at least one gaindevice, wherein the first part of the first channel differential signalis coupled to a fifth switching device in the third conduction path andthe second part of the first channel differential signal is coupled to asixth switching device in the third conduction path, and wherein a thirdcontrol line is coupled to at least one other switching device in thethird conduction path, said third control line providing a controlsignal to turn on the third conduction path to transfer the firstchannel differential signal to the output line with a gain provided bythe at least one gain device in the third conduction path.
 3. Thecircuit according to claim 2 wherein one of the at least one gain devicein the first conduction path and the at least one gain device in thethird conduction path provides positive gain to the first channeldifferential signal and the other at least one gain device provides nogain to the first channel differential signal.
 4. The circuit accordingto claim 2 wherein one of the at least one gain device in the firstconduction path and the at least one gain device in the third conductionpath provides attenuation to the first channel differential signal. 5.The circuit according to claim 2 further comprising a fourth conductionpath coupling the current source and the output line, said fourthconduction path including a plurality of switching devices and at leastone gain device, wherein the first part of the second channeldifferential signal is coupled to a seventh switching device in thefourth conduction path and the second part of the second channeldifferential signal is coupled to an eighth switching device in thefourth conduction path, and wherein a fourth control line is coupled toat least one other switching device in the fourth conduction path, saidfourth control line providing a control signal to turn on the fourthconduction path to transfer the second channel differential signal tothe output line with a gain provided by the at least one gain device inthe fourth conduction path.
 6. The circuit according to claim 5 whereinone of the at least one gain device in the second conduction path andthe at least one gain device in the fourth conduction path providespositive gain and the other at least one gain device provides no gain.7. The circuit according to claim 1 wherein the first control device iscoupled to a fifth and a sixth switching device in the first conductionpath, and the second control line is coupled to a seventh and an eighthswitching device in the second conduction path.
 8. The circuit accordingto claim 1 wherein the switching devices are bipolar transistors, andwherein the signal coupled to a bipolar transistor is coupled to a baseterminal of the bipolar transistor.
 9. The circuit according to claim 1wherein the gain devices are degenerative resistors.
 10. The circuitaccording to claim 1 wherein the current source includes two bipolartransistors and two resistors.
 11. The circuit according to claim 1wherein the first channel is a primary channel and the second channel isa diversity channel in a diversity receiver.
 12. A multiplexer andswitched gain circuit for multiplexing a plurality of analog inputsignals, said circuit comprising: a plurality of input lines responsiveto the analog signals; an output line; a current source providing asource of current, and a plurality of conduction paths coupling thecurrent source and the output line, each conduction path including aplurality of switching devices and at least one gain device, wherein aninput line is coupled to one of the switching devices in each conductionpath, and wherein a control line is coupled to another switching devicein each conduction path, said control line selectively providing acontrol signal to turn on the switching device in the conduction path totransfer the analog signal on the input line to the output line with again provided by the at least one gain device in the conduction path.13. The circuit according to claim 12 wherein the at least one gaindevice in each conduction path provides one of either no gain, positivegain or attenuation.
 14. The circuit according to claim 12 wherein thegain devices are degenerative resistors.
 15. The circuit according toclaim 12 wherein the switching devices are bipolar transistors and thesignal applied to the bipolar transistor is applied to a base terminalof the transistor.
 16. The circuit according to claim 12 wherein theanalog signals are differential analog signals.
 17. The circuitaccording to claim 16 wherein the differential analog signals are from aprimary channel and diversity channel of a diversity receiver.
 18. Thecircuit according to claim 12 wherein the current source includes twobipolar transistors and two resistors.
 19. A multiplexer and switchedgain circuit for multiplexing differential analog signals from a primarychannel and a diversity channel in a diversity receiver, both of theprimary channel and the diversity channel analog signals including afirst part and a second part, said circuit comprising: an output line; acurrent source for providing a source of current, a first primarychannel conduction path coupling the current source and the output line,said first primary channel conduction path including a first bipolartransistor responsive to the first part of the primary channeldifferential signal at its base terminal, a second bipolar transistorresponsive to the second part of the primary channel differential analogsignal at its base terminal, a third bipolar transistor and a fourthbipolar transistor both coupled to a first control line at their baseterminal, said first primary channel conduction path further including afirst degenerative resistor, said first control line providing a controlsignal to turn on the first primary channel conduction path to transferthe primary channel differential analog signal to the output line with again provided by the first degenerative resistor; a second primarychannel conduction path coupled to the current source and the outputline, said second primary channel conduction path including a fifthbipolar transistor responsive to the first part of the primary channeldifferential signal at its base terminal, a sixth bipolar transistorresponsive to the second part of the primary channel differential analogsignal at its base terminal, a seventh bipolar transistor and an eighthbipolar transistor both coupled to a second control line at their baseterminal, said second primary channel conduction path further includinga second degenerative resistor, said second control line providing acontrol signal to turn on the second primary channel conduction path totransfer the primary channel differential analog signal to the outputline with a gain provided by the second degenerative resistor; a firstdiversity channel conduction path coupling the current source and theoutput line, said first diversity channel conduction path including aninth bipolar transistor responsive to the first part of the diversitychannel differential signal at its base terminal, a tenth bipolartransistor responsive to the second part of the diversity channeldifferential analog signal at its base terminal, an eleventh bipolartransistor and a twelfth bipolar transistor both coupled to a thirdcontrol line at their base terminal, said first diversity channelconduction path further including a third degenerative resistor, saidthird control line providing a control signal to turn on the firstdiversity channel conduction path to transfer the diversity channeldifferential analog signal to the output line with a gain provided bythe third degenerative resistor; and a second diversity channelconduction path coupling the current source and the output line, saidsecond diversity channel conduction path including a thirteenth bipolartransistor responsive to the first part of the diversity channeldifferential signal at its base terminal, a fourteenth bipolartransistor responsive to the second part of the diversity channeldifferential analog signal at its base terminal, a fifteenth bipolartransistor and a sixteenth bipolar transistor both coupled to a fourthcontrol line at their base terminal, said second diversity channelconduction path further including a fourth degenerative resistor, saidfourth control line providing a control signal to turn on the seconddiversity channel conduction path to transfer the second diversitychannel differential analog signal to the output line with a gainprovided by the fourth degenerative resistor.
 20. The circuit accordingto claim 19 wherein the first and third degenerative resistors providepositive signal gain and the second and fourth degenerative resistorsprovide no signal gain.
 21. The circuit according to claim 19 whereinthe current source includes two bipolar transistors and two resistors.22. A multiplexer for multiplexing analog signals, said multiplexercomprising: an output line; a first signal path responsive to a firstanalog signal, said first signal path including a first amplifier foramplifying the first analog signal, said first signal path being coupledto the output line; and a second signal path responsive to the firstanalog signal, said second signal path being coupled to the output line,wherein the multiplexer selectively outputs the first analog signal tothe output line through the first or second signal path.
 23. Themultiplexer according to claim 22 wherein the first analog signal is adifferential analog signal.
 24. The multiplexer according to claim 22wherein the second signal path does not provide signal amplification.25. The multiplexer according to claim 22 further comprising a thirdsignal path responsive to a second analog signal, said third signal pathincluding a second amplifier for amplifying the second analog signal,said third signal path being coupled to the output line, and a fourthsignal path responsive to the second analog signal, said fourth signalpath being coupled to the output line, wherein the multiplexerselectively outputs the first or second analog signal to the output linethrough the first, second, third or fourth signal path.
 26. Themultiplexer according to claim 25 wherein the fourth signal path doesnot provide signal amplification.
 27. The multiplexer according to claim25 wherein the first and second analog signals are differential analogsignals.
 28. The multiplexer according to claim 25 wherein themultiplexer is employed in a diversity receiver, where the first analogsignal is a primary channel analog signal and the second analog signalis a diversity channel analog signal.
 29. A multiplexer for multiplexinganalog signals, said multiplexer comprising: an output line; a firstsignal path responsive to a first analog signal, said first signal pathincluding an amplifier for amplifying the first analog signal, saidfirst signal path being coupled to the output line; and a second signalpath responsive to a second analog signal, said second signal path beingcoupled to the output line, wherein the multiplexer selectively outputsthe first or second analog signal on the output line.
 30. Themultiplexer according to claim 29 wherein the amplifier includes adegenerative resistor.